Operational performance of electronic devices are reported to degrade with their exposure to ionizing radiation (Andrew Holmes-Siedle and Leonard Adams. “The Development of an MOS Dosimeter for use in Space”. IEEE Trans. Nucl. Sci., 25 (6), pp. 1607-1612, 1978; N. Bhat and J. Vasi, “Interface-state generation under radiation and high-field stressing in RNO MOS capacitors,” IEEE Trans. Nucl. Sci., 39, 2230, 1992.). Radiation effects are a matter of grave concern for space environment, nuclear applications and electronic gadgets operating in radiation prone environment. These radiation effects can be used in the positive sense as application of these devices in radiation dosimetry. (L. J. Asensio et. al., “Evaluation of a low-cost commercial mosfet as radiation dosimeter”, Sensors and Actuators, A 125, pp. 288-295, 2006.). Use of PMOS transistor in dosimeter was first demonstrated by experiments on Explorer-55. MOS dosimeters are used in the spacecraft, radiation therapy and personal dosimetry. (Andrew Holmes-Siedle and Leonard Adams, “The Development of an MOS Dosimeter for use in Space”. IEEE Trans. Nucl. Sci., 25(6), pp. 1607-1612, 1978.); L. J. Asensio et. al., “Evaluation of a low-cost commercial MOSFET as radiation dosimeter”, Sensors and Actuators, A 125, pp. 288-295, 2006; L. Adams and A. Holmes Siedle, “The development of an MOS dosimetry Unit for use in space”, IEEE Trans. Nucl. Sci. NS-25, pp. 1607-1612, 1978; Meinhard Knoll et. al., “MOS dosimeter and method of manufacturing the same”. European Patent EP0158588, Kind Code: A2).
Ionizing radiation has its application in the field of medical radiation therapy as a part of cancer treatment to control malignant cells. Radiation therapy is commonly applied to the tumor. For the cure, the malignant cells and cells in the close proximity are exposed to ionizing radiation for different dosage depending on the diagnosis. Highly accurate and online measurement of radiation dosages are primary requirement. Personal dosimetry is an important field where measurement of dose of ionizing radiation is done to know how much of radiation is felt by the person when one is in the ionizing radiation ambient. It is desirable that radiation dosimeters for such application are highly accurate, sensitive, portable, flexible, mechanically robust, able to give online reading, and are cheaper.
Inorganic semiconductor detectors are used in medical therapy and personal dosimetry. They are based on crystalline inorganic semiconducting materials like silicon or germanium which absorb energy from ionizing radiation because of which the covalent bonds in their crystalline structure are broken resulting in free electrons and positive holes in the place of the electrons. Pairs of electron-holes contribute in the generation of a current pulse due to exposure of the detector to ionizing radiation. The current pulse is a measure of the ionizing radiation. (Hiroshi Kitaguchi, Kensuke Amemiya, “Semiconductor radiation detectors and apparatus”, European Patent, EP 1584748 A2.). The sensitivity and reliability of the detector varies with temperature because resistivity of any semiconductor decreases with increase in temperature. As the temperature increases due to the thermal energy itself the covalent bonds may break generating electron-hole pairs. As a result, at higher temperatures, the effect of ionizing radiation is not as effectively sensed as at low temperatures. The cost of the detector is also very high because of the inorganic silicon semiconductor technology being costly.
Scintillation counters are also used in medical therapy and personal dosimetry. A scintillation counter comprises a sensor, called a scintillator, which is a transparent crystal, usually phosphor, plastic. (usually containing anthracene), or organic liquid that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube (PMT) measures the light from the crystal which is a measure of the ionizing radiation. The PMT is attached to an electronic equipment to count and possibly quantify the amplitude of the signals produced by the photomultiplier. (G. F. J. GARLI, “The Physics of Scintillation counter”, Journal of Scientific Instruments, vol. 35, 1955; P. F. Hinrichsen, “A stabilized Scintillation counter”, IEEE Trans. Nucl. Sci., 1964; J. M. Fontbonne et. al., “Scintillation fiber dosimeter for radiation therapy accelerator”, IEEE Trans. Nucl. Sci., Vol. 49, No. 5, 2002; Leonid G Korobchenko, Sergei I. Prokofiev, “Liquid Scintillation Counter”, U.S. Pat. No. 4,634,869, 1987; Nurmi et. al., “Liquid Scintillation Counter”, U.S. Pat. No. 4,687,935, 1987.). The scintillation counter requires complicated detection circuit to sense radiation on the basis of photons collected. This in turn requires a PMT which has to effectively collect and amplify small amount of fluorescence generated because of ionizing radiation. It also requires a precision low noise amplifier to amplify it to a detectable value. It is also bulky to realize in a badge form for personal dosimetry and expensive.
Thermoluminescent dosimeter (TLD) is the most popular dosimeter for personal dosimetry applications. A TLD measures ionizing radiation exposure by measuring the amount of visible light emitted from a crystal made up of materials like, lithium fluoride (LiF), lithium borate (LiBO4), calcium fluoride (CaF3), or calcium sulphate (CaSO4). As the radiation interacts with the crystal it causes electrons in the crystal's atoms to jump to higher energy states, where they stay trapped due to impurities (usually manganese or magnesium) in the crystal, until heated. On heating the detector crystal the amount of light emitted is measured by a photomultiplier tube to give a current output proportional to the light sensed. The magnitude of the current is proportional to the radiation exposure. (Frank H Attix et. al., “Thermoluminescent Dosimeter”, U.S. Pat. No. 3,484,605, 1969.). Immediate or real time read out is not possible with the TLD as it has to be heated to give the radiation dose output. It may cause accidental or unintentional release of trapped electrons in the thermoluminescent material prior to read out by exposure to heat, or light, particularly ultraviolet, thereby creating measurement errors. Humidity can also affect the working of the TLD and degrade efficiency thereof.
Organic semiconducting materials are generally used for making organic electronic devices like organic light emitting diodes (OLEDs), organic photo voltaic cells (OPVs) and organic field effect transistors (OFETs). These electronic devices are presently receiving significant attention in organic electronics because of their potential applications in digital switches, backplanes for flat panel displays or radio frequency identification tags (J M Shaw, P F Seidler, “Organic Electronics: Introduction”, IBM J. RES. & DEV. Vol. 45 No. 1, pp. 3-9, 2001; C D Dimitrakopoulos, D J Mascaro, “Organic thin film transistors: A review of recent advances”, IBM J. RES. & DEV. Vol. No. 1, pp. 11-29, 2001; T. W. Kelley, L. D. Boardman, T. D. Dunbar, D. V. Muyres, M. J. Pellerite, and T. P. Smith, “High-Performance OTFTs Using Surface-Modified Alumina Dielectrics,” J. Phys. Chem. B., vol. 107, pp. 5877-5881, 2003; Christos D. Dimitrakopoulos and Patrick R. L. Malenfant, “Organic Thin Film Transistors for Large Area Electronics”, Advanced Materials, Volume 14, Issue 2, pp. 99-117, 2002; H. Klauk, M. Halik, U. Zschieschang, F. Eder, D. Rohde, G. Schmid, and C. Dehm, “Flexible Organic Complementary Circuits,” IEEE Trans. on Elect. Dev., vol. 52, pp. 618-622, 2005). Use of organic semiconductor material is also explored for vapor sensing applications and chemical sensing applications. (D A Thomas et. al., “Organic Semiconductor Sensor Devices”, U.S. Pat. No. 7,141,839, B2, 2006; Heny Wohltjen et. al., “Organic Semiconductor Vapor Sensing Method”, U.S. Pat. No. 4,572,900, 1986.).